1. Field of the Invention
The present invention relates generally to a circuit used for suppressing the pop noise, and more particularly, to a circuit with three-stage of power-on sequence having a soft start device and two identical operational amplifier applied in the audio system.
2. Background
In many audio systems, the output of an audio device is coupled to a speaker via a capacitor, typically referred to as a direct current (DC) blocking or alternating current (AC) coupling capacitor. Usually, the output of an audio device consists of an audio signal and an associated DC offset voltage. Prior to turning on the audio device, the voltage across the AC coupling capacitor is typically zero (0) Volt. When the audio device is turned on, the audio device charges the AC coupling capacitor to the associated DC offset voltage.
The charging of the AC coupling capacitor produces a rising voltage that typically has frequency components within the human audible range. These frequency components typically produce undesirable noise at the output of the speaker, which is typically referred to in the relevant art as “click and pop” noise. Similarly, when the audio device is turned off, the charge on the AC coupling capacitor decays producing a falling voltage that typically also has frequency components within the human audible range. Again, these frequency components produce undesirable click and pop noise at the output of the speaker. This is better explained with reference to the following example
“Pop” noise is also noticeable in other situations. One such situation is in the event of a brief power-down of the amplifier. Typically, one or more capacitors couple the speaker to the audio amplifier, to keep DC components from the amplifier output from propagating to the speaker. If this capacitance discharges too quickly on power-down, a “pop” effect in audible frequencies can be generated. Conversely, if this capacitance does not fully discharge on power-down, remaining charge on the capacitors upon the next power-up of the amplifier can cause a voltage surge that is sufficient to “pop” the speaker.
U.S. Pat. No. 5,537,081, issued to Naokawa et al. entitled “No turn-on pop noise amplifier” discloses an audio amplifier circuit in which the audio amplifier has an input stage that operates at zero common-mode input voltage, and has an output stage to provide the low voltage end of a voltage swing near ground. This reference also discloses a shunt circuit system to discharge external capacitors, preventing “pop” noise from residual charge at power-on. U.S. Pat. No. 5,642,074, issued to Ghaffaripour et al. entitled “Audio transient suppression device” discloses a feedback resistor of the amplifier is first bypassed during power-on period, and then restored once the reference signal of the amplifier is charged up to a predetermined voltage, so as to switch the gain of the amplifier to reduce the pops. U.S. Pat. No. 6,040,740, issued to Dondale, entitled “Audio transient suppression device” discloses a method of lessening or suppressing a transient voltage comprising the step of biasing an output node for an output signal. As shown in FIG. 1, the invention also includes a device for lessening a transient of an amplifier comprising a switch coupled to an output of the amplifier and a bias voltage source, wherein the switch is controllable to bias the output prior to the amplifier providing an amplified audio signal. The resistance of the switch can be variable to adjust the lessening of, or suppress, the transient. Instead of the switch, an impedance device can be used. The invention contemplates lessening or suppressing the transient by either adjusting the charge rate or the bias voltage of the node. Alternatively, both the charge rate and bias voltage can be adjusted to lessen or suppress the transient. This patent changes the gate voltage of device 240 to vary the charge rate of device 220 to suppress pop noise.
However, in order to lessen the pop noise, there is needed to provide a move effective method.